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examBoard: Pearson Edexcel
examType: IGCSE
lessonTitle: Carbon Cycle Stages
The carbon cycle is one of Earth's most important nutrient cycles. It describes how carbon atoms move through the environment, changing form as they travel between the atmosphere, living organisms, soils, oceans and rocks. Carbon is the building block of life - it's in your DNA, proteins and every cell in your body!
Key Definitions:
Carbon is essential for life on Earth. It forms the backbone of organic molecules like carbohydrates, fats, proteins and DNA. Carbon also helps regulate Earth's temperature as carbon dioxide (CO₂) in the atmosphere traps heat. Without the carbon cycle, life as we know it couldn't exist!
Carbon is stored in different places called reservoirs:
• Atmosphere (as CO₂): 800 gigatonnes
• Living organisms: 550 gigatonnes
• Oceans: 39,000 gigatonnes
• Fossil fuels and sedimentary rocks: 100,000,000+ gigatonnes
The carbon cycle involves several key processes that move carbon between different reservoirs. Let's explore each stage:
Plants, algae and some bacteria capture carbon dioxide from the atmosphere through photosynthesis. Using energy from sunlight, they convert CO₂ and water into glucose (a carbon-based sugar) and oxygen.
6CO₂ + 6H₂O + light energy → C₆H₁₂O₆ + 6O₂
This process removes about 120 gigatonnes of carbon from the atmosphere each year, storing it in plant tissues like leaves, stems and roots.
When animals eat plants, they take in carbon-containing compounds. The carbon moves up the food chain as animals eat other animals. This transfers carbon from plants to animals, where it becomes part of their bodies.
Animals use the carbon from their food to build their bodies. Carbon becomes part of proteins, fats and other molecules that make up animal cells and tissues. A typical human body contains about 18% carbon by mass!
Plants store carbon in their stems, leaves, roots and fruits. About half of a plant's dry weight is carbon! Trees are especially important carbon stores - a single mature tree can hold tonnes of carbon in its wood.
Both plants and animals perform cellular respiration, breaking down glucose to release energy. This process returns carbon dioxide to the atmosphere.
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy
Notice that respiration is essentially the opposite of photosynthesis. Together, these processes help maintain balance in the carbon cycle.
When plants and animals die, decomposers (bacteria and fungi) break down their remains. This releases some carbon back to the atmosphere as CO₂, while some carbon becomes part of the soil.
Microscopic decomposers that break down dead matter and release CO₂ through respiration.
Release enzymes to digest dead organisms externally, then absorb the nutrients and release CO₂.
Animals like worms and insects that help break down dead matter, making it easier for microbes to decompose.
The oceans and atmosphere constantly exchange carbon dioxide. CO₂ from the air dissolves in seawater, forming carbonic acid (H₂CO₃), which can further break down into bicarbonate and carbonate ions.
This process is described by the equation:
CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻ ⇌ 2H⁺ + CO₃²⁻
As more CO₂ dissolves in the oceans, the water becomes more acidic (lower pH). Since the Industrial Revolution, ocean pH has dropped from 8.2 to 8.1, which might not sound like much but represents a 30% increase in acidity! This affects marine organisms, especially those with calcium carbonate shells or skeletons like corals and shellfish.
In the oceans, tiny plant-like organisms called phytoplankton use photosynthesis to capture carbon dioxide dissolved in seawater. This carbon then moves through the marine food web, from phytoplankton to small animals to larger predators.
When marine organisms die, some sink to the deep ocean. This "biological pump" transports carbon from surface waters to the deep ocean, where it can remain for hundreds or thousands of years.
Some marine organisms build shells or skeletons using calcium carbonate (CaCO₃). When they die, these structures can form sediments on the ocean floor, storing carbon for millions of years.
Under certain conditions, dead organisms don't fully decompose. Over millions of years, heat and pressure transform these remains into fossil fuels like coal, oil and natural gas - locking away carbon for extremely long periods.
This process happens in stages:
When we burn fossil fuels for energy, we release carbon that has been locked away for millions of years. This rapidly returns carbon to the atmosphere as CO₂.
Human activities release about 9 gigatonnes of carbon into the atmosphere each year, primarily through burning fossil fuels. Only about half of this is absorbed by natural carbon sinks (like oceans and forests). The rest accumulates in the atmosphere, contributing to climate change.
Carbon can be stored in rocks through several processes:
The carbon cycle connects all the Earth's systems - the atmosphere, biosphere (living things), hydrosphere (water) and lithosphere (rocks). Carbon moves between these systems at different speeds:
Carbon moving between atmosphere, plants, animals and soil (days to decades)
Carbon exchange between atmosphere and oceans (decades to centuries)
Carbon moving between atmosphere, oceans and rocks (millions of years)
For your IGCSE exam, make sure you can describe how carbon moves between the atmosphere, living organisms, oceans and rocks. Focus on the key processes: photosynthesis, respiration, decomposition, combustion and ocean exchange. Be able to explain how human activities are affecting the carbon cycle.
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